1. Field of the Invention
The present invention relates to a microcombustion heater having a diameter of 1 to a few centimeters.
2. Description of the Related Art
In a process for heating an object contained in a glass tube so as to produce a semiconductor or the like, the glass tube may be surrounded by several small heaters so as to form a suitable panel heater, and the heating rate of each small heater is controlled in order to control the temperature distribution of the object to be heated.
If such small heaters are of a combustion type (i.e., combustion heaters), it is generally difficult to realize stable combustion. Therefore, small electric heaters are used for manufacturing semiconductors.
On the other hand, small combustors called Swiss-roll combustors are known, which will be explained below.
FIG. 5A is a diagram showing the structure of a one-dimensional counterflow combustion heat exchanger. In the figure, reference numeral 1 indicates a heating wall, and reference numeral 2 indicates a heat insulating wall. The heating wall 1 has a high heat transfer capability, and the heat transfer capability of the heat insulating wall 2 is lower than that of the heating wall. Therefore, when these walls are made of the same material, the heating wall 1 is made thinner and the heat insulating wall 2 is made thicker.
The arrow indicated by reference numeral 3 designates the direction of the flow of a premixed gas of a fuel gas and the air, and the arrow indicated by reference numeral 4 indicates the direction of the flow of the combustion gas. Reference numeral 6 indicates the inlet of the premixed gas, and reference numeral 7 indicates the outlet of the combustion gas. The direction of the flow of the premixed gas and the direction of the flow of the combustion gas, between which the heating wall 1 is arranged, are opposite to each other. That is, the premixed gas is heated via the heating wall 1 by the combustion gas and is then burnt in the combustion chamber 8, thereby producing flame 9.
Reference numeral 10 indicates a passage for the premixed gas, and the reference numeral 11 indicates a passage for the combustion gas. The width of the premixed gas passage 10 is a specific quenching distance (explained below) or less. Therefore, the flame 9 does not flow backward through the passage 10 even when the temperature of the passage 10, heated by the combustion gas, is considerably increased.
In addition, a spark plug (not shown) for starting the combustion is provided in the combustion chamber 8.
Below, the quenching distance will be explained.
As is known, combustion is a heating phenomenon which is caused by a reaction between oxygen in the air and a fuel and which produces a combustion gas. If a gaseous fuel is used, the gaseous fuel reacts with oxygen, thereby producing a flame. The flame is produced due to an immediate reaction between oxygen and the combustion gas as free radicals, which are produced at a high temperature. The flame causes a chain reaction and thus is immediately transmitted. However, the free radicals are deactivated when they collide against a solid wall; thus, the flame cannot be transmitted through a narrow passage between solid walls. The quenching distance indicates a limit width of a narrow gap, that is, the flame cannot flow through a gap if the width of the gap is the quenching distance or less. The quenching distance relates to the combustion speed which depends on the kind of the premixed gas.
The following Table 1 indicates the relationships between the combustion speed of the premixed gas and the quenching diameter (which is used for a passage having a circular section). If the shape of the cross section of the passage is not circular, the following equivalent diameter may be used.Equivalent diameter=(cross-sectional area of narrow gap)×4/(sum of all sides of section of the gap)
TABLE 1Combustion SpeedQuenching DiameterPremixed Gas(cm3/cm2/s)(mm)Propane-Air45.72.7Hexane-Air39.63.0Ethylene-Air70.11.9Acetylene-Air176.80.79Hydrogen-Air335.30.86
FIG. 5B is a sectional view of a two-dimensional combustor 5, called a Swiss-roll combustor, produced by arranging the passage of the above-explaining one-dimensional combustor so as to have a spiral form, where the combustion chamber 8 is positioned at the center of the spiral.
The Swiss-roll combustor 5 is a regenerative heat exchanger which has a small heat loss even though the device size is small. The premixed gas is preheated before the combustion, so that reliable combustion can be realized and combustion can be performed even with a lean premixed gas which has approximately one-third of the specific theoretical fuel-air ratio (or the equivalent ratio).
As explained above, small heaters which are generally used for heating semiconductors are electric heaters, and the electric heater consumes expensive electrical energy which is converted into thermal energy. Therefore, this type of heater is not economical.
On the other hand, the above-explained Swiss-roll combustor 5 is a combustor which was not anticipated to be used as a device having a heating surface, where a target object is heated by radiant heat emitted from the heating surface.